1. Field of the Invention
The present invention relates to a lighting control unit for a vehicle lighting fixture and in particular to a lighting control unit designed to control lighting of a semiconductor light source including a semiconductor light-emitting device.
2. Description of the Related Art
In the related art, a vehicle lighting fixture is known that uses a semiconductor light-emitting device such as an LED (Light Emitting Diode). This type of vehicle lighting fixture mounts a lighting control circuit for controlling lighting of an LED.
A lighting control circuit has been proposed in which for example a forward type switching regulator is used as a switching regulator to control lighting of an LED (refer to JP-A-2004-8409, page 4, FIG. 2). The forward type switching regulator has a transformer and a switching element connected to the primary side of the transformer. The forward type switching regulator is designed to emit electromagnetic energy accumulated on the primary side of the transformer while the switching element is ON to the secondary side of the transformer and supply the emitted electromagnetic energy to the LED via a rectifier circuit.
The maximum value of the switching regulator is determined by an input voltage as the battery voltage of a vehicle and the winding ratio of a transformer. For example, when the input voltage is 13V and the winding ratio of a transformer is 1:4, the output voltage is 13V×4=52V. The winding ratio of a transformer is set considering variations in the battery voltage. Since the battery voltage of a vehicle varies with engine operating state or load state. Thus, it is requested that an LED be illuminated despite variations in the range of 6 to 20V. As a result, when the forward voltage of the LED is about 30V, the winding ratio of the transformer should be 1:5 or over (6V×5=30V−(20V×5)=100V) in order to stably illuminate the LED. When the winding ratio of the transformer is set to 1:6 with some margin considered, the output voltage is 36V for an input voltage of 6V (6V×6=36V) thus stably illuminating the LED. When the battery voltage reaches 20V, the output voltage of the switching regulator reaches 20V×6=120V. A break of wire in the LED at this time elevates the output voltage of the switching regulator to 120V or more. This makes it necessary to use an LED with a pressure resistance that will not go faulty when the output voltage of the switching regulator exceeds 120V, which increases costs.
In order to prevent the increase in cost, it is possible to use a configuration where the output voltage of the switching regulator is monitored and the switching element is turned off when the output voltage of the switching regulator has exceeded a set voltage, for example 54V.
JP-A-2002-8409 (Page 4, FIG. 2) is referred to as a related art.
When too low a set voltage is specified in the configuration where the switching element is turned off when the output voltage of the switching regulator has exceeded the set voltage, variations in Vf (forward voltage) of an LED may result or the LED may fail to illuminate depending on the corresponding temperature characteristic. In particular, when an LED with a high Vf is used at low temperatures, it may cause the LED to illuminate despite a wire break, which will lower the system safety.
When too low a set voltage is specified to turn off the switching element, the transformer may be saturated in a process where the output voltage of the switching regulator rises.
A transformer used for a forward-type switching regulator has its primary side and secondary side generally coupled to each other while the switching element is turned on and a rise in the current flowing through the transformer is suppressed by the inductance of a coil inserted in the secondary side of the transformer. Thus, the transformer need not have a gap and coupling efficiency is upgraded by possibly eliminating a gap. When the coupling of the primary side and the secondary side is in good condition, there is no danger of magnetic saturation. When the secondary side of the transformer is open with the wire break in the LED, the transformer has no energy delivery port on the secondary side thus resulting in magnetic saturation at a lower current value caused by the absence of the gap unless the primary side and the secondary side are coupled again.
Immediately after the LED has encountered a wire break, the output voltage of the switching regulator is low so that the transformer can deliver its energy to a smoothing capacitor on the secondary side. In the process where the output voltage of the transformer rises in the absence of coupling of the primary side and the secondary side of the transformer, the primary side of the transformer merely serves as a coil with the absence of a gap, which results in magnetic saturation at a lower current value.
When magnetic saturation has taken place, the current flowing on the primary side of the transformer suddenly rises. This could damage the switching element without proper action being taken. In this case, to suppress magnetic saturation of the transformer, the set voltage may be set to a value considering variations in the Vf of the LED and temperature characteristic and the duty ratio of a switching signal applied to the switching element may be lowered with the rise in the output voltage of the switching regulator.
However, it is difficult to match the degree of lowering the duty ratio of the switching signal with the degree of magnetic saturation of the transformer. Too high a degree of lowering the duty ratio of the switching signal could result in variations in the Vf of the LED or the LED could fail to illuminate depending on the temperature characteristic. This could impair the safety of the system.
Another approach is to turn off the switching element to limit the current when the current flowing through the transformer has suddenly risen on occurrence of magnetic saturation. In the process, the energy of the current is applied to the switching element with the timing the switching element is turned off. The energy could damage the switching element when the switching element consumes the suddenly elevated current at the time of wire break, leading to power loss.